The present invention relates to an electron gun for a color picture tube which is capable of freely varying an electron beam emitting angle and a focus voltage ratio with respect to a high-voltage to improve focusing and voltage endurance characteristics.
Referring to FIG. 1, a conventional color picture tube comprises a glass bulb having a panel 2 whose inner surface is covered with fluorescent film 8 to produce a beam spot when an electron beam impacts thereon, and having a funnel 3 around which a deflection yoke 9 is mounted, an electron gun 5 which produces and emits electron beam 6 for R, G and B colors to the panel 2, with sealed in a neck portion 4 of the glass bulb, and a shadow mask 7 which is installed to the inner surface of the panel 2 for the color selection.
To describe the electron gun 5 for the conventional color picture tube in detail, referring to FIG. 2, it comprises three cathodes 10 which emit an electron beam 6 for each of R, G and B colors, being arranged in an in-line manner and first to sixth grid electrodes 11 to 16 which adjust an amount and focusing of the electron beams 6 emitted from the cathodes 10, being supported by bead glass 17 of regular insulator and arranged in the electron beam emitting direction.
The first to the sixth grid electrodes 11 to 16 will be described in detail according to their shapes, applied voltages, and operations.
The first, the second and the fourth grid electrodes 11, 12 and 14 are of the plate shape, while the third, the fifth and the sixth grid electrodes 13, 15 and 16 are of the cap shape.
To the fourth and the sixth grid electrodes 14 and 16 a high voltage Eb is commonly applied so that the electron beams may be accelerated, and to the third and the fifth grid electrodes 13 and 15 a focus voltage Vf, corresponding to 28% or 33% of the high voltage Eb, is applied so that the focusing of the electron beams can be adjusted. And gaps between adjacent grid electrodes of the conventional electron gun are as follows:
______________________________________ gap ______________________________________ 1st electrode - 2nd electrode 0.25-0.3 mm 2nd electrode - 3rd electrode 0.8 mm 3rd electrode - 4th electrode 1.0 mm 4th electrode - 5th electrode 1.0 mm 5th electrode - 6th electrode 1.0 mm ______________________________________
In a conventional electron gun having such a configuration, the procedure that a picture image is produced from the electron gun will be described.
The electron beams 6 emitted from the cathodes 10 are adjusted in an amount thereof through the first grid electrode 11, and then accelerated in the second grid electrode 12, they enter the third to the sixth grid electrodes 13 to 16 which formulates main and supplementary electrostatic lenses, thereby the focus of the electron beams 6 is adjusted and accelerated so that they impact on the fluorescent film 8 to produce a beam spot.
Such a beam spot passing through the shadow mask 7 which is color selection electrode, its color is selected, and then scanned on the screen by the deflection yoke 9 mounted around the funnel 2 so that the picture can be produced.
In such a conventional electron gun as mentioned above, the high voltage Eb applied to the fourth and the sixth grid electrodes 14 and 16 and the focus voltage Vf applied to the third and the fifth grid electrodes 13 and 15 are limited by the thickness of the fourth grid electrode 14. Furthermore, since, in a currently generally applied electron gun, the focus voltage to high voltage ratio Vf/Eb is 28% or 33%, the thickness of the fourth grid electrode 14 is to be adopted to each case. Thus, the gap between the fourth and the fifth grid electrodes 14 and 15 is also limited.
Accordingly, since the focus voltage of high voltage ratio Vf/Eb is limited in accordance with the thickness of the fourth grid electrode 14, it is limited to reduce the potential difference between the adjacent grid electrodes, the gap between the adjacent grid electrodes is to be 1.0 nm. As a result, when passing through the grid electrodes, the electron beams may be scattered by colliding with an alien substance, lying between the adjacent electrodes, therefore, a picture image is not made on the screen and there happens to be a stray, which is a phenomenon the blue-fluorescence appears between the electron gun 5 and the neck portion 4 of the glass bulb. Due to the stray, flaking of the surface of cathode and short-circuiting of a heater may occur. Furthermore, the higher the high voltage Eb is, the more frequently the stray phenomenon may occur.